The present invention relates to an image forming apparatus, such as a reader-printer for projecting an enlarged image of a microfilm to allow the observation thereof by the operator and for producing a hard copy of the microfilm through an electrophotographic process, a printer wherein a photosensitive member is exposed to a light image provided by a computer or the like, by way of laser beam scanning, LED elements, to provide a hard copy of the light image, and an ordinary electrophotographic machine.
The original image of a microfilm is either positive or negative. However, a hard copy thereof is desired to be positive, irrespective of whether or not the original is positive, so that the hard copy is easily read. Therefore, a reader-printer is preferably capable of producing a positive image copy from a positive original and from a negative original. Heretofore, there has been no such apparatus that the mode can be easily switched between a mode wherein a positive copy is obtained from a positive original (hereinafter called "P-P mode") and a mode wherein a positive copy is obtained from a negative original (hereinafter called "N-P mode").
In machines, such as a reader-printer and an electrophotographic copying machine, which use an electrophotography, the area of a photosensitive member which is not exposed to the light image, retains the electric charge. Such an area of the photosensitive member attracts the developing toner during the developing operation in the P-P mode, that is, the area is developed. The development of the unnecessary area wastes the toner. It is usual, in order to prevent this, that a uniform light is projected onto the surface of the photosensitive member when the light image is not projected thereonto. That is a so-called blank exposure.
In the N-P mode development (reversal development), on the other hand, the developing toner is deposited on such area of the photosensitive member as has been exposed to light, but the area not exposed to the light, that is, the area still retaining the electric charge, does not attract the developing toner. Therefore, if the above described blank exposure is effected in the N-P mode development, the developing toner is deposited onto the area exposed to the light by the blank exposure. Namely, the toner is deposited on the non-image area, which has not been exposed to the image light. The deposited toner is not transferred to a transfer material, but simply removed from the photosensitive member and consumed wastefully.